Effort is being made to improve automobile headlamps by making headlamps with small cross sections to reduce wind resistance and thereby enhance vehicle mileage. By generating light more efficiently, electrical demands may also be reduced, again enhancing mileage. By increasing lamp durability, vehicle maintenance, and warranty service costs are also reduced. A reduced light source size may also enhance optical accuracy in forming a projected beam. Light quality may then be improved, enhancing vision, without increasing glare or stress to oncoming drivers. All of these advantages may be achieved with a low wattage arc discharge headlamp. Low wattage arc discharge lamps, however, are not sufficiently well developed to be quickly adapted to vehicle use. Further development of arc discharge lamps is needed to make a practical vehicle lamp. In particular, there is a need for an arc lamp envelope shape for direct current operation, minimal warmup time, and horizontal operation to produce about 70 lumens per watt at about 30 or 35 Watts.
Different electrode structures have been investigated in search of a proper design for direct current operation. Merely adjusting electrode shapes has not produced the features needed in a practical vehicle lamp. The shape of the capsule must also be adjusted, particularly in the region adjacent the cathode, the negative electrode. The cathode end of the arc produces a larger portion of the light, and is therefore placed at or near the focal point of a reflector. Variations in the arc dynamics, particularly those adjacent the cathode, then have a substantial affect on the beam. Proper placement of the cathode, and its interaction with the envelope are therefore recognized as important to overall beam quality. Placement of the anode, and the interaction with the adjacent lamp wall is less critical for photometric performance, but still essential for proper heat transfer.
Examples of the prior arc discharge lamp art are shown in U.S. Pat. Nos. 3,259,777; 4,161,672; 4,170,746; 4,396,857; 4,594,529 and 4,779,026.
Elmer Fridrich U.S. Pat. No. 3,259,777 issued July 5, 1966 for Metal Halide Vapor Discharge Lamp with Near Molten Tip Electrodes shows tubular shaped arc discharge lamps. FIGS. 2a, 3a, 4 and 5 show small tubular lamps
Daniel Cap et al. U.S. Pat. No. 4,161,672 issued on July 17, 1979 to for High Pressure Metal Vapor Discharge Lamps of Improved Efficacy discusses the shapes and electrode penetrations of lamps of less than 250 watts. In particular, Cap discloses a 30 watt ellipsoidal lamp with an internal volume of 0.066 cm.sup.3 having a diameter of 3.5 millimeters, and a length of 4.5 millimeters. Cap is concerned with nearly spheroidal to elongated spheroids in combination with electrodes inserted from 4.55 to 18.75 percent of the long diameter.
John Davenport U.S. Pat. No. 4,170,746 issued on Oct. 9, 1979 for High Frequency Operation of Miniature Metal Vapor Discharge Lamps discusses the operation of spherical lamps with 3.2, 4.0, 5.0, 6.0, and 7.0 millimeter internal diameters operated at different alternating current frequencies.
George Danko U.S. Pat. No. 4,396,857 issued on Aug. 2, 1983 for Arc Tube Construction shows a miniature discharge tube having a volume from 0.1 to 0.15 cm.sup.3. Danko claims the use of cylindrical solid neck portions adjacent the bulbous central volume. The cylindrical neck portions help assure a surface of revolution around the longitudinal axis of the lamp.
Bertus de Vrijer U.S. Pat. No. 4,594,529 issued on June 10, 1986 for Metal Halide Discharge Lamp discloses a miniature tubular arc discharge lamp. de Vrijer is concerned with the tubular dimensions of a lamp for use as a headlamp.
Jurgen Heider U.S. Pat. No. 4,779,026 issued on Oct. 18, 1988 for Rapid Start High Pressure Discharge Lamp and Method of Its Operation shows a miniature arc discharge lamp with a tubular body, and slightly pinched transitions between the seals and bulb region. Heider discusses lamps with volumes less than 0.03 cm.sup.3.